Control valve and method

ABSTRACT

A delay valve particularly well suited for use in nuclear reactor hydraulic control systems and a control method employing the valve are disclosed. The valve includes a bellows attached to a plug member and a high resistance flow path between a first pressure source and the interior of the bellows whereby a predetermined time is required for each filling of the bellows and the accompanying movement of the plug member in a first direction. Fission rate control devices are positioned in response to delay valve operation which is controlled by the application of pressure from second and third sources to the exterior of the bellows in the proper sequence.

[ 1 Oct. 15,1974.-

5/1952 Norton et al. 91/417 x ABSTRACT A delay valve particularly well suited for use in nuincludes a bellows attached to a plug member and a high resistance flow path between a first pressure source and the interior of the bellows whereby a predetermined time is required for each filling of the bellows and the accompanying movement of the plug member in a first direction. Fission rate control devices are positioned in response to delay valve operation which is controlled by the application of pressure from second and third sources to the exterior of the bellows in the proper sequence.

5 Claims, 3 Drawing Figures 0 United States Patent [191 Groves [541 CONTROL VALVE AND METHOD 3033.170

[75] Inventor: glcarllclolm D. Groves, Simsbury, Primary Examiner lrwin C. Cohen [73] Assignee: Combustion Engineering, lnc., [57] Windsor, Conn.

[22] Filed: Mar. 20, 1972 clear reactor hydraulic control systems and a control H pp No 236 462 method employin the valve are disclosed. The valve 91/166,9l/417,9l/468, 251/54 [51] Int. FlSb 11/08, F15b 13/042 [58] Field of Search....... 91/38, 417, 235, 321, 468,

[56] References Cited UNITED STATES PATENTS 2,815,042 12/1957 Passaggio.......................... 251/48 X A. a 13,- 1| II CONTROL VALVE AND METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the control of nuclear reactors. More specifically, this invention is directed to a delay valve for use in a reactor hydraulic control system. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.

2. Description of the Prior Art While not limited thereto in its utility, the present invention is particularly well suited for use in and as a hydraulic system for controlling the position of absorber elements relative to the fuel assembly of a nuclear reactor. Reactor hydraulic control systems are well known in the art. A typical control system provides for the raising of absorber elements, in the interest of increasing fission rate and reactor power output, in response to a pressure differential established across a lift piston connected to the absorber element. Reactor internal coolant pressure is constantly applied to one side of the lift pistons and an absorber element withdrawal may therefore be accomplished by the application of a lower pressure to the other side of a selected lift piston.

Since pressure vessel internal pressure is substantially higher than atmospheric pressure, the rupture of a hydraulic control line externally of the vessel will result in the creation of a sufficient pressure differential across the lift piston to cause absorber element withdrawal. In order to guard against an undesired control rod withdrawal resulting from a control line rupture,

the prior art has typically employed hydraulic fuses installed in each control line immediately downstream of the lift piston cylinder. The hydraulic fuse is a valve device which will automatically close the flow path if a preselected flow rate, indicative of a control line rupture, is obtained or exceeded. I

There are a number of disadvantages incident to the use of a hydraulic fuse. First, the nature of the hydraulic fuse is such that it limits the maximum permissible flow through the control line and thus interfers with the speed and efficiency of normal operation. A further disadvantage of a hydraulic fuse is that it is not responsive to leakage flow rates greater than necessary to actuate the lift piston but less than the rate commensurate with total control line rupture. Thus, there can be undesired absorber element motion without operation of the fuse to close the line experiencing the leakage. Accordingly, the art has long desired a device which would perform in the same manner as a hydraulic fuse but which would not limit maximum permissible lift flow and would achieve more positive control in the sense that absorber element motion could occur only after a first deliberate or positive control action was performed.

As a further disadvantage of the hydraulic fuse, it is to be noted that the fuse is a passive device which may, since there is no practical way to test the device, be unoperated for periods approaching several years. With such passive devices there is the inherent possibility that the valving function will not be accomplished or will only be partially accomplished when needed due to various causes such as, for example, scale build-up.

SUMMARY OF THE INVENTION The present invention overcomes the above briefly discussed and other disadvantages of the prior art by providing an active and positive method and apparatus for controlling the application of a pressure to a hydraulic actuator. Apparatus in accordance with the present invention includes a novel delay valve which, in the environment of a nuclear reactor, will be installed in place of the hydraulic fuses of the prior art.

The present invention contemplates the exercise of control over a hydraulic actuator, such as a piston connected to a reactor absorber element, through performance of a plurality of control operations in the proper sequence. The delay valve of the present invention will be normally closed and the hydraulic actuator controlled thereby will be in a first or unenergized condition. The delay valve is opened by application of a first control pressure thereto. This delay valve control pressure will act in opposition to the pressure which is employed to activate the hydraulic actuator. When the delay valve has been opened application of the valve control pressure is discontinued and a second pressure is applied, through the now open valve, to the hydraulic actuator to cause energization thereof. Although removal of the delay valve control pressure permits the valve to begin to close, the built in time delay is sufficient to enable the actuator to reach its fully energized position before the delay valve fully closes. The actuator is maintained in the energized position as long as the second pressure is applied by means of leakage flow which is permitted to bypass the closed delay valve.

The normally closed delay valve of the present'invention is a bellows operated device wherein, considering the environment of a nuclear reactor control system, reactor pressure vessel coolant outlet pressure is applied internally of the bellows via a restricted flow path between the interior of the bellows and the interior of the pressure vessel. Application of a control pressure greater than that applied internally of the bellows causes compression of the bellows and opening of the valve. The delay valve will not open without the positive step of application of the control pressure to the exterior of the bellows and thus the delay valve will not operate to permit hydraulic actuator energization in the event of a control line break downstream of the valve.

BRIEF DESCRIPTION OF THE DRAWING The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawing wherein like reference numerals refer to like elements in the several figures and in which:

FIG. 1 is a schematic view of a reactor control system employing the present invention;

FIG. 2 schematically illustrates a preferred mode of installation of a delay valve in accordance with the invention; and

FIG. 3 is a cross-sectional side elevation view of a preferred embodiment of a delay valve in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference now to FIG. I, the pressure vessel of a pressurized water reactor is indicated generally at 10. Reactor vessel provides a housing for the various elements of a fission type nuclear reactor of the type employed by utilities to heat a circulating coolant; the coolant thereafter being routed through the heat exchangers and other components of a stream generator and the steam thus provided being employed to drive a turbine and its associated electrical power generator.

In FIG. 1 the steam generators and associated equipment have been labelled PRIMARY SYSTEM and are indicated generally at 12. The main circulating pump for the coolant is indicated at 14 and supplies, via conduit or cold leg 16, the coolant to pressure vessel 10. The heated coolant exits from the pressure vessel 10 via conduit or hot leg 18 and is thereafter delivered to PRIMARY SYSTEM 12.

As is well known in the art, a core assembly including a plurality of fuel rods or elements is positioned within pressure vessel 10. Also positioned within vessel 10, in the interest of controlling the fission rate, are control or neutron absorber rod assemblies; a typical such assembly being indicated schematically and generally at 20 and including an absorber element 21. For a detailed description of a top actuated reactor control system of the type shown schematically in FIG. 1 reference may be had to copending Application Ser. No. 211,308 filed Dec. 23, 1971 and assigned to the assignee of the present invention. The disclosure of copending Application Serial No. 21 1,308 is incorporated herein by reference.

While not limited thereto in its utility, the delay valve of the present invention is particularly well suited for use in the top actuated system of the type described in referenced application 21 1,308. In accordance with such a top actuated system each of the control rod assemblies has two operative positions commensurate respectively with full retraction and full insertion of its absorber element into the reactor core. As shown in FIG. 1, absorber element position control is acheived by providing each individual control rod assembly with a hydraulic actuator. The position of the absorber elements is determined by the application of pressure, considering a top actuated system for purposes of explanation, to the top end of each of the hydraulic actuators. A plurality of hydraulic control line, such as lines 22-22 associated with control rod assembly 20, will penetrate the pressure vessel in order to provide for the application of the control pressure to the individual hydraulic actuators.

Externally of the pressure vessel a control valve 24 will be installed in each of the control lines 22. One side of each of valves 24 is connected to a lift pump 26 via a manifold 28. As may be seen from the flow path indicated in broken lines in FIG. 1, primary coolant delivered to pressure vessel 10 will flow through the individual control rod assemblies and any coolant drawn off by lift pump 26 will be returned via conduit 29 to the main coolant flow path upstream of the main circulating pump 14.

Continuing with a general discussion of reactor operation, when all of the absorber elements are in the up position, as shown in the case of control rod assembly 20 of FIG. 1, the reactor will be operating with maximum power output. The adjustment of reactor output power may thus be accomplished by controllably inserting or withdrawing individual absorber elements from the core.

As noted above, in accordance with the scheme of referenced Application Ser. No. 211,308 each of the control rod assemblies 20 will include a hydraulic actuator. The details of such a hydraulic actuator may be seen from FIGS. 3, 3A and 3B of copending Application 211,308. The selective raising of individual absorber elements is accomplished by opening appropriate valves 24 whereby a lift pressure differential is established across the pistons, such as piston 30, of the hydraulic actuators associated with the selected absorbers. Insertion of selected individual absorber elements into the core under normal operating conditions is accomplished by operating valves 24 so as to remove holding pressure from the tops of the selected actuators. Upon the removal of the holding pressure absorber element insertion may be achieved solely under the influence of gravity, by the application of pressure to the tops of the hydraulic actuators or by gravity with a positive pressure assist. If power or power assisted control rod insertion is desired valve 24 will be a threeway valve and a supplemental source of pressure, not shown, will typically be utilized.

As noted above, the delay valve of the present invention prevents absorber element withdrawal in the event of an external control line rupture and permits a more positive mode of control when compared to the prior art. In order to maximize the safety enhancement provided by the invention, the valve is installed in control line portion 22' within the reactor pressure vessel and as close to the hydraulic actuator portion of the control rod assembly as practical. In FIG. 1 the delay valve is indicated schematically at 32. As will become obvious from the discussion below, two pressure sources in addition to pressure vessel internal or outlet pressure are necessary for proper operation of the control system of the present invention. The first or low pressure additional source is provided by lift pump 26 via control valve 24 and control line 22-22'. The second or high pressure additional source may be derived from the cold leg 16 and applied via a supplemental control valve 34 installed in a conduit 36 which provides communication between cold leg 16 and control line 22. The derivation of the high operating pressure from the cold leg 16 has been described by way of illustration only and it is to be understood that any other supplemental pressure source which provides pressure higher than that maintained within the reactor pressure vessel may be employed. Thus, referring to FIG. 2, such a supplemental pressure source has been indicated at 38 while the low pressure source, as provided by pump 26 and manifold 28, has been indicated at 40.

Before discussing the structure and operation of the disclosed preferred embodiment of the present invention, the actual installation of the delay valve in a reactor pressure vessel will be described. Thus, with reference to FIG. 2, the delay valve 32 is shown mounted on the upper guide structure top support plate 42. Delay valve 32 is installed in internal control line portion 22 as close to the control rod assembly as practical. The external portion 22 of the control line is directed through the wall of the pressure vessel, via a penetration device not shown, and communication is established with pressure sources 38 and 40 by respective valves 34 and 24. Internally of the pressure vessel means, in the form of a disconnect mechanism 44, are provided for separating the control line portions during reactor head removal for maintenance purposes. Disconnect mechanism 44 may be of the type shown in FIGS. 5 and 5A of copending Application Ser. No. 211,308. The disconnect mechanism is supported on an upper guide structure flange 46 which is in turn mounted on top of the reactor core support barrel 48.

Referring not to FIG. 3, a preferred embodiment of a delay valve in accordance with the present invention is indicated generally at 32. Valve 32 is shown mounted on upper guide structure top support plate 42 directly above a control rod assembly with the interior of the valve in direct communication with the cylinder of the hydraulic actuator portion of the control rod assembly. Restated, the piston 30 (FIG. 1) of the hydraulic actuator moves within the cylinder indicated at 50. Valve 32 comprises an upper portion 52, which functions as a bellows housing, and a lower portion 54. In the embodiment of FIG. 3 the lower portion 54 of valve 32 also functions as a top buffer assembly which, in cooperation with a flow restricting portion of the piston 30, prevents mechanical impact and water hammer pulsations by providing flow reduction which causes deceleration of the control rod in such a manner that flow through control line 22 is substantially cut off prior to the seating of the piston 30 against a top seat 56 defined by valve portion 54. The operation of such a top buffer is described in the discussion of FIG. 3B of copending application 21 1,308.

Valve portion 54 defines a main flow path 58 between the seat 56 and an upper seat which cooperates with the delay valve plug member 60 to perform the valving function. Valve portion 54 is also provided with a hold up flow channel 62 which provides communication between cylinder 50 and the chamber 64 in which plug member 60 moves; chamber 64 also being defined by valve portion 54. It is to be noted that the system tolerances are such that flow through hold up channel 62 will continue with piston 30 positioned against seat 56. Channel 62 is sized to permit the flow necessary to hold the absorber element-lift piston assembly in the raised position. Channel 62 is also sized or restricted to the degree necessary to prevent sufficient flow to cause absorber upward motion in case of a control line break. Communication between control line 22' and chamber 64 is acheived via a control port 66 formed in the side of valve portion 54 and a fluid tight joint between the control line and valve portion 54 may be accomplished by any suitable means.

The upper portion 52 of valve 32 is comprised of an outer cylinder defining tubular member 68 which is joined at it lower end to the top of valve portion 54 whereby the interior of member 68 defines an extension of chamber 64. As shown in FIG. 3 cylinder defining member 68 will typically be provided with an external thread which will engage an internal thread provided above a shoulder in valve portion 54. A seal will be provided on the internal shoulder where the end of tubular member 68 abuts portion 54 and a weld will be provided about the exterior of member 68 at the junction with the top of valve portion 54.

Valve portion 52 will be provided with a cap member 70 which is threadably engaged in tubular member 68.

Cap member has, either formed integrally therewith or attached thereto by means such as welding, an elongated guide member 72 which extends downwardly therefrom. Guide member 72 is coaxial with tubular member 68 and is provided with a central bore 74 which defines a cylinder; bore 74 communicating with the lower end of member 72. Fluid communication between the bore 74 and the interior of the pressure vessel is provided by a channel 76 which extends through the top of cap 70. Thus, as may be seen from a joint consideration of the FIGS. 2 and 3, the reactor outlet pressure will be applied to the interior of bore 74 for the purposes to be described below.

A rod 78 is disposed for movement within bore 74 of guide member 72. Rod 78 extends out of bore 74 and is joined, at its lower end, to the plug member 60. The rod 78 is provided with a series of expansions and contractions which define, for the purpose to be explained below, a high resistance flow path to pressure vessel coolant entering bore 74 via channel 76. A bellows 80 is sealably attached, at its first end, to plug member 60. The opposite or upper end of bellows 80 is sealably attached to the exterior of guide member 72 at a convenient location; typically adjacent the junction of member 72 and cap 70. Accordingly, vessel discharge pressure will be applied to the interior of bellows 80 and the movement of bellow 80 and thus the operation of the valve will be controlled by the pressure applied to the exterior portion of the bellows which is disposed in chamber 64. 7

Operation of the present invention and particularly of delay valve 32 will now be described. The delay valve will be normally closed, as shown in FIG. 3, and it will be presumed that the absorber element associated therewith is fully inserted into the reactor core. At this time pressure vessel outlet pressure will be applied both internally and externally of bellows 80. Bellows 80 is a spring bellows and thus plug member 60 will be positively seated at this time. Valve 34 will be opened to unbalance the delay valve when it is desired to withdraw the absorber element. The opening of valve 34 will apply a pressure higher than the vessel outlet pressure to chamber 64 via control line 22'. This control pressure, which will typically be 25 psi higher than the pressure internally of bellows 80, will cause the bellows to be compressed thereby opening the delay valve by moving plug 60 upwardly. After the delay valve has been opened valve 34 will be closed and control valve 24 opened. The opening of valve 24 permits normal and maximum lift flow through the open delay valve thereby causing the hydraulic actuator piston and the absorber element attached thereto to move upwardly. The full upward stroke of the absorber element will typically take from 20 to 40 seconds and the lift flow during this period will typically be in the range of 20 gallons per minute. During the absorber element lift cycle the plug 60 is prevented from closing by the inherent time delay built into the valve. That is, when the valve is opened the bellows 80 is compressed and the coolant expelled therefrom. The sizing of channel 76 in cooperation with the high resistance path defined by the expansions and contractions in rod 78 defines a high resistance path to flow which impedes the refilling of the inside of bellows 80 thereby imparting a sufficient time delay to permit complete raising of the absorber element.

When the absorber element is fully withdrawn the piston 30 will seat against surface 56 and the flow through the delay valve will drop from the lift flow value to a small leakage flow which passes around the piston and through hold up flow channel 62. When plug 60 is reseated, after the delay built into the device, the leakage flow through channel 62 will hold the absorber element in the raised position as long as valve 24 remains open and communication is established between chamber 64 and the low or lift pressure source. Absorber element insertion may be accomplished by closing valve 24 to interrupt the holding pressure whereupon the weight of the actuator piston-absorber element assembly will be sufficient to reinsert the absorber element in the core.

To summarize operation of the present invention, the normally closed delay valve can be opened only by the performance of a first positive control step of application of a pressure to the exterior of the delay valve bellows which is greater than that applied to the interior of the bellows. Such a high or control pressure will not be applied in the case of a control line break. With the delay valve closed, and the absorber element either raised or inserted in the core, flow through the delay valve is limited to that permitted by channel 62. The flow through channel 62 is insufficient to move the absorber element-lift piston assembly but is sufficient, with the low lift pressure source applied to the delay valve, to hold the absorber element in the raised position. The absorber element is raised during the time period, determined by the design of the delay valve, it takes the bellows to refill after the positive delay valve control pressure has been removed. Since it is only during this time period that the maximum flow necessary to develop the lift pressure differential can occur, a control line rupture externally of the pressure vessel will not cause lift flow rate to be accidentally established and thus will not cause undesired absorber element movement.

While a preferred embodiment has been shown and described various modifications and substitutions may be made without departing from the spirit and scope of the invention. Accordingly it is to be understood that the present invention has been described by way of illustration and not limitation.

What is claimed is:

l. A method of controlling the position ofa hydraulic actuator, said actuator including a movable piston, a pressure which biases the actuator piston for movement in a first direction being normally applied to a first side of the piston, said method comprising the steps of:

applying a first pressure which opposes said bias pressure to a normally closed pressure responsive control valve, said first pressure causing opening of the control valve; terminating application of the first pressure; applying a second pressure which aids said bias pressure to the second side of the actuator piston via the open control valve, the second pressure cooperating with the bias pressure to provide a pressure differential across the actuator piston of sufficient magnitude to induce movement of the actuator piston, the second pressure permitting closing of the control valve; imparting a delay to the closing of the control valve after termination of application of the first pressure thereto; and maintaining a holding flow for the actuator piston by application of the second pressure thereto via a restricted flow path after closing of the control valve.

2. The method of claim 1 further comprising: biasing the pressure responsive control valve in the closed direction by application thereto of a pressure intermediate said first and second pressures. 3. The method of claim 2 wherein the control valve includes a bellows having a first side thereof exposed to said first and second pressures and wherein the step of imparting aclosing delay comprises:

applying said intermediate pressure to the second side of the bellows via a restricted flow path. 4. The method of claim 3 further comprising: terminating application of said second pressure when it is desired to return the actuator to its initial position whereby the holding flow will be discontinued.

5. The method of claim 3 wherein the step of applying said intermediate pressure to the second side of the bellows comprises:

establishing permanent fluid communication between the second side of the bellows and the source of bias pressure for the actuator piston. 

1. A method of controlling the position of a hydraulic actuator, said actuator includIng a movable piston, a pressure which biases the actuator piston for movement in a first direction being normally applied to a first side of the piston, said method comprising the steps of: applying a first pressure which opposes said bias pressure to a normally closed pressure responsive control valve, said first pressure causing opening of the control valve; terminating application of the first pressure; applying a second pressure which aids said bias pressure to the second side of the actuator piston via the open control valve, the second pressure cooperating with the bias pressure to provide a pressure differential across the actuator piston of sufficient magnitude to induce movement of the actuator piston, the second pressure permitting closing of the control valve; imparting a delay to the closing of the control valve after termination of application of the first pressure thereto; and maintaining a holding flow for the actuator piston by application of the second pressure thereto via a restricted flow path after closing of the control valve.
 2. The method of claim 1 further comprising: biasing the pressure responsive control valve in the closed direction by application thereto of a pressure intermediate said first and second pressures.
 3. The method of claim 2 wherein the control valve includes a bellows having a first side thereof exposed to said first and second pressures and wherein the step of imparting a closing delay comprises: applying said intermediate pressure to the second side of the bellows via a restricted flow path.
 4. The method of claim 3 further comprising: terminating application of said second pressure when it is desired to return the actuator to its initial position whereby the holding flow will be discontinued.
 5. The method of claim 3 wherein the step of applying said intermediate pressure to the second side of the bellows comprises: establishing permanent fluid communication between the second side of the bellows and the source of bias pressure for the actuator piston. 